This invention concerns methods and apparatus for injection molding of synthetic resinous material. More particularly, the present invention is concerned with vented injection molding of degradable synthetic resinous materials without predrying.
In the past, injection molding of hygroscopic synthetic resinous materials was frequently preceded by drying of the particulate feed material. The drying is effected by placing a batch of the particulate feed material in suitable drying apparatus and causing warm dry gas to circulate around the particulate material. Circulation of the warm dry gas removes moisture and lowers the water content of the feed material. The dried material is then fed into injection molding machine where it is plasticized, pressurized and formed into a useful product.
The predrying of hygroscopic material, however, is time consuming since it adds an additional step to the injection molding process and ordinarily proceeds as a batch type process. Moreover, the temperature of the heated gas which circulates through and around the particulate material must be maintained at a sufficiently low level that the synthetic particulate material does not become molten or tacky. Even with the constraint on the available drying gas temperature level, the predrying apparatus may cause some discoloration of the material when dealing with a material such as nylon.
In addition to the foregoing, the expense of a separate predrying step is also significant in the injection molding or articles. For example, the predrying apparatus requires floor space in addition to that required by the injection molding machine and involves additional labor costs in order to handle the individual batches of feed material.
An alternative to the separate predrying apparatus in combination with an injection molding machine has been suggested. The alternative comprises an injection molding machine having a barrel that is provided with a vent through which steam and other devolatilized gaseous products are exhausted from the inside of the barrel to the ambient atmosphere or to suitable treatment apparatus. While a vented injection molding machine may be effective to restrict the amount of predrying required for some hygroscopic synthetic resinous feed materials, it exhibits other difficulties when used in commercial production of articles fashioned from degradable materials such as nylons. More particularly, after many hours of continuous cyclic operation, known vented injection molding machines yield articles that include a multiplicity of black specs.
With continued production, the number of black specs increases and eventually the molded products are no longer suitable for commercial use. When this occurs, the injection molding machine must be disassembled to clean residue from the flights of the injection molding machine screw. The periodic disassembly and cleaning are expensive in terms of money and time and, therefore, are commercially undesirable.
Accordingly, the need continues to exist for a vented injection molding machine which is capable of continuously producing injection molded articles from synthetic resinous nylon materials without requiring the disassembly and cleaning of the injection molding machine at relatively frequent intervals.